Landfill gas contains a large number of trace hydrocarbon impurities and water which must be removed prior to the bulk separation of methane from carbon dioxide, which separation is generally carried out in a pressure swing adsorption (PSA) system. In order to obtain effective CO.sub.2 /CH.sub.4 separation in the PSA system the water and hydrocarbon impurities need first to be removed from the crude LFG, since these impurities adversely effect the separation capacity of the selective adsorbent, such as zeolite, employed in the PSA system. If not removed the impurities end up in the product methane.
In a known previously utilized practice crude landfill gas was pretreated in a thermal swing adsorption (TSA) system to remove the water and hydrocarbon impurities prior to subjecting the LFG to separation of CO.sub.2 /CH.sub.4 by PSA. The impurity-laden TSA adsorbent was regenerated, using part of the pretreated CO.sub.2 and CH.sub.4 -containing stream or the eventual CH.sub.4 product stream to heat the adsorbent. The same gas was used to cool the TSA adsorbent before a new cycle could be started. This procedure results in the loss of a significant amount of the cleaned feed gas (CO.sub.2 +CH.sub.4) or of the valuable separated product gas (CH.sub.4).
U.S. Pat. No. 4,000,990 discloses a landfill adsorptive separation process wherein a closed cycle regenerated pretreatment system is used in conjunction with a pressure swing adsorptive bulk separation of carbon dioxide and methane. The process of that patent suffers from the inefficiencies of c1osed 1oop regeneration of pretreatment beds wherein only gas contaminated by trace impurities is used to remove similar impurties from an offstream regenerating pretreatment bed. The result is that in that patent when a predetermined is brought back on-stream a certain level of residual impurities is carried over to the bulk separation pressure swing adsorption beds which are subsequently contaminated with impurities requiring periodic thermal regeneration resulting in disruption of the overall process and requiring large energy expenditures.
U.S. Pat. No. 4,329,158 discloses a process for separation of nitrogen from oxygen wherein a pretreatment adsorptive separation is performed prior to the bulk separation of the major constituents of air. Nitrogen enriched waste gas is utilized from the bulk separation portion of the process to regenerate the pretreatment portion of the process. The bulk separation of nitrogen from oxygen is performed with an elevated temperature adsorption of nitrogen, a desorption of bulk separation beds to a lower pressure, a purge of the beds with product oxygen after desorption countercurrently and two steps of repressurization to elevated pressure first with waste gas which is nitrogen enriched and secondly with product oxygen.
U.S. Pat. No. 4,077,779 discloses a process wherein methane and carbon dioxide can be resolved in a six step adsorptive pressure swing process including the steps of adsorption, high pressure rinse, depressurization, inert gas rinse, evacuation and repressurization.
In an article titled "Pressure Swing Adsorption for Natural Gas or Methane Recovery from Landfill Gas" hy E. Richter. et al., a process is disclosed which uses carbon molecular sieve or silica gel to separate carbon dioxide from methane in a biogas feed stream using pressure swing adsorption including the steps of pressure build-up adsorption expansion and evacuation.
In a literature publication by Bergbau-Forschung a process for methane recovery from a carbon dioxide-containing biogas stream is dismethane closed wherein pressure swing adsorption is used to perform the separation with a carbon molecular sieve. The steps include adsorption, depressurization. evacuation and pressure build-up.
In an article titled "A New Process for the Production of High BTU Gas" by W. R. Koch of Mar. 17, 1986, a process is disclosed wherein pretreatment of landfill off-gas is performed by adsorption followed by a bulk separation of methane from carbon dioxide and a pressure swing adsorption procedure including the steps of adsorption, high pressure rinse. depressurization, evacuation and repressurization. Details of pretreatment are not set forth.
In a sales brochure titled "Landfill Gas Treatment Experience With The GEMINI 5 System" by K. F. Potochnik, et al. of 1987, a process is set forth wherein landfill gas is treated in a pressure swing adsorption process including a pretreatment to remove minor contaminants and a bulk separation of carbon dioxide from methane using adsorption, high pressure rinse, depressurization, evacuation and repressurization. The pretreatment beds are recited to be regenerated with a regeneration gas which is thermally heated. Thereafter, the pretreatment bed is cooled down by a passage of a cool gas stream through the regenerated bed.
In yet another sales brochure titled "Landfill Gas Purification With The GEMINI 5 System" No. 522-604, a process for treating landfill gas is disclosed including the steps of feed compression and drying, a pretreatment stage which removes trace impurities for thermal combustion and venting followed by a pressure swing adsorption system which removes carbon dioxide for vent while retaining methane as a product stream for product compression.
Additional art of only general interest to the concept of adsorptive separation of gas mixtures include: U.S. Pat. Nos. 3,594,983; 3,751,878; 4,013,429; 4,249,915; 4,264,340; 4,314.828; 4,153.428; 4,472,178.